The present invention relates to a process for hardening stainless steel, as well as a molten salt bath for realizing this process.
Owing to its excellent corrosion-resistance properties, stainless steel is used for constructing chemical apparatuses, in the field of food technology, in the petrochemical industry for offshore applications, for the ship and airplane construction, in architecture, for constructing houses and technical equipment, as well as for many other industrial applications.
Corrosion-resistant stainless steel is understood to refer to an iron material with at least 13% by weight of chromium added by alloying. In most cases, nickel, titanium and molybdenum are also added to the iron alloy, e.g. as explained in the Steel Instruction Leaflet 821 entitled “EDELSTAHL ROSTFREI-EIGENSCHAFTEN-INFORMATIONSSTELLE EDELSTAHL” [Corrosion-Resistant Stainless Steel-Characteristics-Information Source for Stainless Steel] PF 102205, 40013 Düsseldorf; www.edelstahl-rostfrei.de, and in P. Gümpel et al. “ROSTFREIE STÄHLE” [Corrosion-Resistant Steels], Expert Publishing House, Volume 349, Renningen Malmsheim 1998. Typical austenitic stainless steels are the alloys of steels 1.4301 or 1.4571 and have the following compositions in % by weight:1.4301: 0.05 C; 0.5 Si; 1.4 Mn; 18.5 Cr; 9.5 Ni1.4571: 0.03 C; 0.5 Si; 1.7 Mn; 17.0 Cr; 11.2 Ni; 2.2 Mo; 0.1 Ti.
If the chromium content is less than 13% by weight, the steel in general is not sufficiently corrosion-resistant to be considered stainless steel. The metallic chromium content of the steel therefore represents an important criterion for the corrosion resistance, as explained in P. Gümpel et al. “ROSTFREIE STÄHLE” [Corrosion-Resistant Steels], Expert Publishing House, Volume 349, Renningen Malmsheim 1998.
The fact that most generally used types of stainless steel such as 1.4301, 1.4441, 1.4541, or 1.4575 are rather soft steels and are therefore subject to scratching of the surface by hard particles such as dust or sand is a major disadvantage. Most stainless steels, apart from the very specialized martensitic stainless steels, cannot be hardened by using physical processes such as annealing and quenching. The low surface hardness frequently prevents the use of the stainless steel. Most types of stainless steel furthermore have a tendency to strong adhesion through friction, meaning two surfaces sliding against each other are welded together as a result of adhesion.
The surface of stainless steel can be enriched with nitrogen by subjecting it to a thermo-chemical treatment, e.g. nitriding or nitro-carbureting in gas (in an ammonia atmosphere), in plasma (with nitrogen/argon) or in the molten salt bath (molten cyanate salts), during which iron nitrides and chromium nitrides are formed. In contrast to physically deposited layers or layers formed by electroplating, the resulting layers are formed from the material itself, meaning they are not externally deposited and therefore have extreme adhesive strength. Hard layers with a thickness ranging from 5 to 50 μm are thus formed, depending on the treatment length. The hardness of such nitrided or nitro-carbureted layers on stainless steel reaches values above 1000 units on the Vickers hardness scale because of the high hardness of the resulting iron nitrides and chromium nitrides.
The problem with depositing such nitrided or nitro-carbureted layers on stainless steel in practical operations is that the layers are hard, to be sure, but lose their corrosion-resistance because of the relatively high treatment temperature for the nitriding or nitro-carbureting treatment, which is in the range of 580° C. At this temperature, the diffused-in elements nitrogen and carbon form in the component surface region stable chromium nitrides (CrN) and/or chromium carbides (Cr7C3) together with the chromium. The free chromium, which is absolutely required for the corrosion resistance, is thus extracted from the stainless steel matrix up to a depth of approximately 50 μm below the surface and is converted to chromium nitride or chromium carbide. The component surface is hardened due to the iron nitride and chromium nitride that forms, but also becomes susceptible to corrosion. Such layers are worn down and/or eroded quickly during use as a result of corrosion.
The following methods are currently in use for avoiding this problem.
It is known that the surface hardness of stainless steel can be improved through electroplating, e.g. nickel-plating or depositing of physical layers with the PVD method (physical vapor deposition). These processes, however, require an alien material to be deposited on the steel surface, meaning the steel surface is no longer the surface in contact with the corrosive or abrasive medium. As a result, there are problems with the adhesion and the corrosion-resistance. These processes are consequently not widely used to improve the hardness and corrosion-resistance of stainless steel.
A hard and simultaneously corrosion-resistant layer can be formed with thermo-chemical deposition on stainless steel and using the so-called Kolsterisieren® (kolsterizing process). This process is mentioned, for example, in the information leaflet Kolsterisieren®—Anticorrosion Surface Hardening of Austenitic Corrosion-Resistant Steel—from the company Bodycote Hardiff bv, Parimariboweg 45, NL-7333 Apeldoorn, info@hardiff.de, as well as in M. Wägner, “STEIGERUNG DER VERSCHLEISS-FESTIGKEIT NICHTROSTENDER AUST. STÄHLE” [Improving the Corrosion-Resistance of Non-Rusting Aust. Steels], in “STAHL” [Steel], Issue No. 2 (2004) 40-43. The process conditions are not described either in patent literature or in the scientific literature accessible to the public. Components treated in this way have a hard, wear-resistant layer with a thickness of between 10 and 20 μm while the corrosion-resistance of the basic material is preserved. Components that are Kolsterisiert® (kolsterized) must not be heated above 400° C. since they otherwise loose their corrosion resistance.
Using the plasma nitriding process, for example described in H.-J. Spies et al. “MAT.-WISS. U. WERKSTOFFTECHNIK 30 [Material Knowledge and Material Technology 30] (1999) 457-464, as well as in Y. Sun, T. Bell et al. “The Response of Austenitic Stainless Steel to Low Temp. Plasma Nitriding Heat Treatment of Metals,” Issue No. 1 (1999) 9-16, or the process of vacuum carburization as described, for example, in “OBERFLÄCHENHÄRTUNG VON AUSTENITISCHEN STÄHLEN UNTER BEIBEHALTUNG DER KORROSIONSBESTÄNDIGKEIT” [Surface Hardening of Austenitic Steels while Maintaining Corrosion-Resistance] by D. Günther, F. Hoffmann, M. Jung, P. Mayr in “HÄRTEREI-TECHN. MITT.” [Hardening Technology Information], 56 (2001) 74-83, it is possible to generate an over-saturated solution of nitrogen and/or carbon at low temperatures in the surface of components made from stainless steel. This solution has the desired characteristics, meaning the higher hardness along with unchanged corrosion resistance.
However, both processes require high apparatus expenditure and high investment and energy costs, as well as the use of specially trained personnel, in most cases scientifically trained personnel, for operating the systems.
A process for the case-hardening of rust-resistant steel is known from German Patent Application DE 35 01 409 A1. With this process, the surface of the work piece to be hardened is initially activated by treating it with an acid and is then treated inside a heated fluidized bed containing active nitrogen and preferably also active carbon, capable of diffusing into the work piece.
A process for carburizing austenitic metal is described in German Patent Application DE 695 10 719 T2. According to this process, the metal is heated and kept in a fluorine-containing or fluoride-containing gas atmosphere prior to the carburization. The carburizing of the metal then takes place at a maximum temperature of 680° C.